1. Field of the Invention
The present invention relates to built-in-coil substrates and more specifically relates to built-in-coil substrates including a coil built into the inside of the substrate.
2. Description of the Related Art
Module components in which circuit components such as semiconductor integrated circuits, capacitors, and resistors are mounted on a substrate having a built-in coil, manufactured by alternately stacking insulating layers, which contain a ceramic material, and inner conductor layers on top of one another and firing the integrated multilayer body, are used in cellular phones.
Built-in-coil substrates have been proposed in which cavities are provided inside the substrates.
For example, in Japanese Unexamined Patent Application Publication No. 8-64421, it is disclosed that, regarding a multilayer inductor, in a firing process, magnetic layers and inner conductor layers are integrated into a single sintered body, but while cooling to room temperature, residual stress is generated due to the difference in thermal expansion coefficient between the magnetic layers and the inner conductor layers, and consequently magnetostriction is generated and the performance of the multilayer inductor is reduced. Therefore, a cavity for allowing relaxation of stress is formed outside of the outermost layer among the inner conductor layers.
In Japanese Unexamined Patent Application Publication No. 2005-294725, it is disclosed that by using a first conductive paste, which contains resin particles and has a thermal contraction coefficient equal to or greater than that of ceramic green sheets, and a second conductive paste, which has thermal contraction coefficient smaller than that of the ceramic green sheets, cavities are formed as a result of the resin particles being destroyed by firing and thereby stress is reduced.
In Japanese Unexamined Patent Application Publication No. 2006-352018, it is disclosed that, as illustrated in the sectional view of FIG. 9A and the enlarged sectional view of FIG. 9B, in a case where cavities 140 are formed between coil electrodes 130 inside a multilayer body 110 of a multilayer electronic component, the number of cavities 140 is made to be half or more the number of coil electrodes 130 and the ratio between a width a of the cavities 140 in the stacking direction and a distance b between adjacent coil electrodes 130 in the stacking direction is made to be 0<a/b≦½. The ratio is made to be 0<a/b in order to realize an effect of stress relaxation and the ratio is made to be a/b≦½ in order to realize an effect of preventing cracks.
In International Publication No. 2009/081865, it is disclosed that, by stacking inner coils and outer coils of different sizes, the formation of unevenness on the surface of a multilayer body is suppressed and the occurrence of cracks due to the concentration of stress in magnetic layers on the outermost sides in the stacking direction is suppressed compared with the case where coils having the same diameter are superposed.
A built-in-coil substrate is manufactured by dividing a collective substrate containing portions that will become a plurality of built-in-coil substrates into individual pieces. When cavities are provided inside the collective substrate, fissures occur that originate from the cavities and break defects and cracks are liable to occur when the collective substrate is bent in order to divide the collective substrate into individual pieces.